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The CarterCopter is an experimental compound autogyro developed by Carter Aviation Technologies to demonstrate slowed rotor technology. On 17 June 2005, the CarterCopter became the first rotorcraft to achieve mu-1 (μ-1), an equal ratio of airspeed to rotor tip speed.

Contents

Design and development

The Carter copter is a pusher configuration autogyro with wings and a twin boom tail. The rotor is a two-bladed design weighted with 55 pounds (25 kg) depleted uranium at each tip. It is an all-composite design[2] The tricycle undercarriage is retractable. The undercarriage has a large travel to allow for landing at up to 20 ft/sec.

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Concept

The CarterCopter concept is a gyrocopter with an unusually stiff, relatively heavy rotor, supplemented with conventional wings. At low speed, the vehicle flies as a gyrocopter, and can pre-spin the rotor for a vertical takeoff and very brief hover, and can land more or less vertically.

At high speed (above about 100 mph) the aircraft flies mostly using the fixed wings, with the rotor simply windmilling. The rotor spins with a tip speed below airspeed, which means that the retreating blade flies completely stalled. On a helicopter this would cause massive lift asymmetry and insoluble control issues but the fixed wings keep the aircraft in the air and stable.

The low rotation speed and flat feathering of the rotor means that it causes little drag, and the company claims that the aircraft would be potentially able to leverage the advantages of fixed wings as well as gyrocopters, giving almost all the capabilities of helicopters (except hovering) but with a relatively simple mechanical system. Carter Aviation also claims the system is safer than a typical helicopter.[citation needed] The CarterCopter should be capable of the higher airspeeds that can only be achieved by fixed wing aircraft, but also able to land in any small area in an emergency.

Takeoff

At takeoff the pilot angles the top rotor flat (zero angle of attack) and spins it to very high speed. The rotor is then disconnected from the engine and the angle of attack of the main rotor blades is increased suddenly so that the vehicle leaps into the air. The aircraft's main rotor has enough momentum due to heavy counterweights in the tips that it can hover for a short time safely. The pilot then applies full power to the rear pusher propeller and the vehicle starts to move forwards. As it does so, air is forced through the main rotor, spinning it faster and generating more lift. The vehicle climbs into the air, flying as an autogyro.

Cruising

Once the CarterCopter gets up to a forward speed of about 90 miles per hour (145 km/h), its stubby, lightweight wings provide most of the lift. The pilot can then flatten the angle of attack of the main rotor so it produces very little lift, dramatically reducing the amount of induced drag created by the rotor. Although the rotor is unused at high speed, the rotor is kept spinning as it keeps the rotor rigid, preventing excessive flapping.[3]

Normally a helicopter or gyrocopter cannot fly forward at the same speed (or faster) as its rotor tip speed. This is because the retreating rotor blade would stop in the air, whilst the leading rotor blade would be traveling at twice the speed of the aircraft. The vehicle would 'fall over' due to retreating blade stall and the dissimilar lift.

However, with the CarterCopter, the fixed wings keep the vehicle at the correct angle to the horizon and provide the lift required to remain aloft. Since the rotor is unloaded, the aerodynamic forces on the rotor are very minor. This means that a CarterCopter can theoretically fly much faster than the tip speed of the rotor. The rotors would still experience flapping as they rotate due to dissymmetry of lift lift between the two sides of the vehicle, but Carter Aviation claims this is manageable.

The claimed theoretical maximum speed of a CarterCopter is around 500 mph (800 km/h), which would be about twice as fast as any helicopter has ever gone.

Achievements

At present, the prototype's engine is normally aspirated, and hence is limited to just 320 hp (240 kW) and the fastest Carter Aviation Technologies prototype has achieved is about 173 mph (270 km/h); which is still ~40% faster than a conventional autogyro but slower than gyrodynes of the 1950s.

A helicopter to go the same speed would need almost twice this power.[citation needed] Thus the CarterCopter seems to be about twice as efficient.

From 1999 to 2001 there were 4 recorded instances of non-fatal crashes.[4]

The maximum mu that has been achieved (mu is the ratio of airspeed to rotor tip speed)[5] is 1.0 for a brief moment on June 17, 2005, the first time any rotary aircraft has reached this level. (CarterCopter's pilot claimed that there was no great drama, and mu 1 was reached accidentally due to normal variations in rotor RPM and vehicle airspeed; the pilot described it as 'smooth' with no significant vibration.)

However, on the next test flight the same day, the CarterCopter made a hard landing (crashed), causing significant damage, but the pilot was unhurt. The crash was caused by a partial loss of control of the rotor due to a hardware failure. It was initially believed that the CarterCopter was unrepairable; later inspection showed that it could be repaired, but the company has chosen to work on other projects at this time.

The testing proved[citation needed] that the vehicle architecture could potentially outperform helicopters on every dimension except sustained hover, and should be much cheaper to buy and maintain. It also very nearly matches the L/D of fixed wing General Aviation aeroplanes at cruise speed[6]- but with near-vertical takeoff and landing; and at only modestly higher cost to buy and maintain than a fixed wing aircraft.